There is an immediate and growing need for satellite-based, global communications on-demand, between a hand-held user transceiver and a central gateway or hub. This need, which applies to both the U.S. government and non-government sectors, includes 24 hour-a-day random access communications support for emergency indication, small sensors, law enforcement, and many other "remote" user scenarios. The emphasis is on the ability of the satellite system to accommodate transmissions from hand-held units at anytime and anywhere; communications back to the user must also be supportable, but typically in reaction to a transmission from the user. Such hand-held, random-access communication scenarios cannot be accommodated by existing commercial satellite communication systems, but, according to this invention, can be readily supported by a novel and unique utilization of NASA's Tracking and Data Relay Satellite System (TDRSS) or a similar satellite system. The associated TDRSS support, coupled with the transceiver technology and implementation, are the subjects of this invention.
Most communication satellites operate at geosynchronous altitude, an altitude of about 22,000 miles, at which point the earth's disk appears approximately 20 degrees across. These satellite communication systems have traditionally utilized broad-coverage antennas to concurrently receive signals from, and transmit signals to, regional or near-hemisphere areas, while remaining over a fixed spot on the earth's equator. The broad antenna beam, at typical frequencies (e.g., microwave), corresponds to a small-area transmit-receive antenna. This, in turn, limits the electromagnetic power the antenna can intercept. The result is that, for acceptable communication quality, users on the ground must have relatively large antennas and/or transmit many watts of power; this, in turn, typically leads to transceivers that cannot be hand-held and, further, precludes efficient battery-powered operation.
Typical satellite transponders (that is, the on-board equipment for relaying signals within a given frequency bandwidth) are in essence amplifier-frequency-shifters which can accept signals from any user-transmitter on the ground operating within the band covered, amplify those signals, shift their frequency and retransmit them through another antenna to a central gateway. Since the signals are not demodulated or signal-processed on-board the satellite, there is no processing gain to compensate for low signal power.
Special purpose communication satellites (e.g., for the Department of Defense) have been built for a variety of purposes. With a larger antenna on the satellite, it is possible to communicate with a user on the ground having a correspondingly smaller antenna and/or transmitter power. In this case, however, the beamwidth of the satellite's antenna is reduced, thereby requiring the location of the ground user to be known, and the satellite's antenna tracked to that location. Were the antenna mechanically tracked by rotating itself or the entire satellite, that would use propellant at an unacceptable rate; what's more, it could serve concurrently only users in a small area of the earth. Electronic antenna steering provides a highly attractive alternative that eliminates the disadvantages of mechanical steering while simultaneously providing the ability to focus on many regions concurrently with high gain; electronic steering can also be accomplished much more rapidly than mechanical steering, again without any incurred mechanical satellite motion. Electronic steering is more expensive than traditional non-steerable antennas, and have heretofore appeared mainly on military satellites. Furthermore, even on such military satellites, the number of simultaneous receive beams that can be formed, and their operational flexibility, has been limited by the specific on-board beamforming capability employed. In this regard, the electronic beamforming capability used by the TDRSS is especially unique.
To satisfy its needs for global communications with low earth orbiting spacecraft, NASA has developed the Tracking and Data Relay Satellite Systems (TDRSS), which includes geosynchronous satellites that are able to electronically steer an on-board phased-array antenna. This phased array views the entire earth's disk, but can form many simultaneous beams to support reception of many independent user transmissions; each such beam has a beamwidth considerably narrower than the earth's disk and thus also provides considerably higher gain than an earth coverage beam. Furthermore, this same phased-array can form a single narrow beam at a time to provide high power transmissions back to the user; this beam can be independent of, or directly related to, any of the many simultaneous receive beams. As such, both the receive (inbound) and transmit (outbound) beams are sufficiently powerful to accommodate low-power; hand-held user transceivers; not only is this operationally attractive to the user but it also provides the added benefit of extended battery lifetime and reduced exposure to RF emissions.
Electronic beam steering requires that signals from a number of separate antenna elements, most commonly arranged in a planar area, be phase-shifted by amounts depending on the distance of the element from the center of the array and the direction in which the beam is to form. Whether the application is radar or communications, such antennas typically have their beamforming accomplished at the antenna. In this regard the TDRSS is unique, in that the inbound beamforming is performed on the ground. Specifically, the TDRSS transmits the signal, received by each on-board antenna element, separately to the ground station in a composite, frequency-multiplexed signal. Since the coverage of each element of the TDRSS is more than the angle of the earth's disk, the combination of signals sent to the ground can be combined on the ground to "form" a much narrower beam and to direct it, free of any mechanical inertia. This has several advantages relative to conventional approaches of beamforming at the antenna. First, a beamformer on the ground can be replaced if a failure occurs. Second, the number of independent beamformers can be much greater on the ground than can be possibly placed on-board a satellite. Third, the number of independent beamformers can be expanded, if needed, after the satellite is on orbit. Finally, the beamforming algorithms can evolve and improve with technology, if the beamforming is accomplished on the ground. Clearly, all of these advantages of ground-based beamforming yield a greatly increased satellite "return on investment".
Many global voice and data communication needs, both government and non-government, remain unmet through application of conventional communication satellites. More particularly, few (if any) satellite sensor or communication systems can communicate flexibly (e.g., random access) and successfully with small, low-powered, hand-held, low-data-rate ground-based user transceivers, or with remote instruments or controllers not equipped with large antennas or, equivalently, with high output power.
The object of this invention is to provide an improved global satellite communication system that uniquely applies the TDRSS without impact to its prime mission of supporting low-earth-orbiting science spacecraft. This invention encompasses both the satellite system concept, and the ground-based transceiver design and implementation required for successful system operation.